1. Field of the Invention
The present invention relates to a fluidized bed apparatus and, more particularly, to improved fluidized bed apparatus and methods for processing of very fine particles with gases.
2. Prior Art
A variety of fluidized bed devices and technologies have been proposed and used over the years with fluid-solid systems. The fluidized bed system offers a unique combination of advantages over other types of wet and dry operating processes and applications, particularly where large quantities of solid particles are involved. Such systems are advantageously used where a great amount of heat transfer is desired, where quantities of solid particulate materials must be transported, where there is a need for optimization of chemical reactions by thorough contact between solids and gases and the like. For example, fluidized bed devices are used for solid particle treatments such as drying, coating, oxidation, firing, granulation, etc.
Fluidized bed apparatus generally consists of a chamber or enclosure within which a layer of finely divided solid particles are supported on a bed and such material is "fluidized" by gaseous fluids being passed upward through the mass of particles at a sufficient velocity. If the velocity of the gas is properly adjusted, the solid particles separate and move about in a turbulent random manner such, that the entire bed of solid particles behaves like a liquid.
Full benefit of the advantages of fluidized bed processes depends on uniform fluidization of the solid particles without agglomeration of the small particles, channeling of the gas through the bed and entrainment of particles in the gaseous fluid that is discharged. Heretofore, fluidized bed processing of very fine solid particles, such as fine kaolin clays was not feasible because such extremely fine particles have a tendency to agglomerate and large quantities of the particles become entrained in the discharging gases from which separation was difficult.
The distribution of particle sizes is also an important factor in obtaining even fluidization, the larger particles requiring different gas velocities for fluidization than smaller particles. Since fluidized bed systems must be designed for one gas velocity, very broad particle size distribution in the bed leads to a bed with stagnant large particles and entrained small particles which obviously affect the characteristics of the system and cause it to deviate from the ideal conditions which is, theoretically, attainable and desirable. Thus, materials such as kaolin, where particle sizes are dictated by nature, require prior classification and treatment to permit their processing by fluidized bed techniques.
Uniform dispersion of the gaseous fluid across the mass of particles in the bed is important for optimum performance of the apparatus. In general, this is accomplished by means of a perforated gaseous diffusion plate or construction located at the bottom of the chamber or enclosure, which also typically serves as a support means for the layer of solids to be fluidized and as a separator between the fluidized bed chamber and gas inlet chamber. The diffusion-separator is generally constructed and arranged to diffuse the gas flowing through the bed, uniformity being obtained by having sufficient pressure drop resistance through each opening in the separator and with the openings arranged to require approximately equal flow rates across the entire separator area. In addition, a suitable diffusion-separator must be constructed so as to permit little or no passage of solid particles from above the diffusion-separator into a plenum chamber where the fluidization gas enters the apparatus.
A variety of diffusion-separator configurations are known, varying from simple metallic plates having small holes drilled therethrough or specialized porous metal and ceramic plates to more involved fluidized bed structures having diffusion plates with bubble caps, perforate baffles, pipes, capillary tubes, vibratory excitation agitation means and the like.
For example, in U.S. Pat. No. 3,161,483, vibratory excitation is applied to conveyors and reactors involved in fluidized bed treatment; in U.S. Pat. No. 2,840,923, vibration generation means are applied to perforated channels used for drying materials in granular form; in U.S. Pat. No. 3,768,174, vibratory excitation of a fluidized bed device having a perforated sheet metal separator is used for drying and/or cooling fine granular products; in U.S. Pat. No. 3,511,843, vibratory means are used to agitate cohesive materials being treated with a gas to maintain the cohesive material in a fluidized state; in U.S. Pat. No. 3,618,227, vibratory means are used in the discharge hopper of a continuous drying chamber for resins to prevent blocking off the distribution of gas; in U.S. Pat. No. 4,235,024, vibratory means are used in conjunction with a gas diffusion plate of particular configuration to impart a circulatory motion to a fluidized bed of solid particles; in U.S. Pat. No. 4,305,210, vibratory means are employed to keep fluidized particles on a thin perforated corrugated diffusion plate advancing through the processing stages; and in U.S. Pat. No. 4,323,312 is disclosed a rotating disc device for directing the flow of fluidized particles in a fluidized bed apparatus.
While vibratory means used in combination with a variety of diffusion plate configurations in fluidized bed devices is suggested as a means of preventing agglomeration and channeling of gas through a bed of solid particle as well as reducing the gas velocity requirements to achieve fluidization, there still exists the problem of entrainment of very fine solid particles in the discharged gas. Further, the known systems are not flexible enough to be useful with solid particles having a wide distribution of particle sizes.
In U.S. Pat. No. 4,068,389, there is disclosed a gas diffusion plate for fluidized bed apparatus which provides improved means for regulating the gas velocity through each of the vents in the plate and for preventing solid particles from flowing back into the entrance gas plenum. While such a diffusion plate achieves an improvement in control of gas velocities over the whole fluidized bed area and in the dispersion of the fluidizing gas, it does not completely overcome the problem of agglomeration and entrainment of very small-size particles. Moreover, no means are suggested that would be flexible enough for use with solid particles having a wide particle size distribution.
As would be evident, entrainment of very finely divided particles in the discharging gas is a significant problem since recovery of the particles from the gas is important to avoid loss of material and environmental problems. However, separation of very fine solid particles from the gas is generally difficult and expensive. As disclosed, for example, in U.S. Pat. No. 3,161,483, cyclone separators are typical of the type of apparatus used to recover entrained particles from discharging fluidization gases in fluidized bed apparatus. A variety of other types of apparatus have been suggested over the years for removing solid particles entrained in flowing gases, such as disclosed, for example, in U.S. Pat. Nos. 905,999, 1,082,356 and 4,304,753. None of these, however, are concerned with fluidized bed techniques or suggest means that would be applicable in such devices.
It is apparent that the development of fluidized bed apparatus which could be effectively used with extremely finely divided solid particles such as kaolin clays would be highly desirable. It would be particularly desireable if such apparatus was flexible enough to be used with solid particles having a wide distribution of particle sizes and/or provided means for readily recovering entrained particles from discharging fluidization gases.